Ion-exchange membrane electrodialytic salt production using brine discharged from a reverse osmosis seawater desalination plant

Abstract

Operating parameters of an ion-exchange membrane electrodialytic salt manufacturing plant (NaCl production capacity: 200,000 t per year) using brine discharged from a reverse osmosis (RO) seawater desalination plant are discussed. The results were compared with the data obtained from a salt manufacturing plant using seawater. The specifications of the electrodialyzer are: the thickness of the desalting cell, 0.05 cm; the flow-pass length in a desalting cell, 2 m; effective membrane area, 2 m 2; overall osmotic coefficient of a membrane pair, 30 cm 4/(eq. h); and solution velocity at the inlets of desalting cells, 5 cm/s. The electrolyte concentration at the inlets of desalting cells was set at 1.5 eq./dm 3, which is consistent with the electrolyte concentration of brine discharged from a reverse osmosis seawater desalination plant. The energy consumed in the salt manufacturing process was assumed to be supplied by a simultaneous heat-generating electric power unit using a back-pressure turbine. The number of evaporators (evaporation pans) was selected to minimize the electric power shortfall of the salt manufacturing process but to be larger than zero. The electric power shortage was assumed to be made up by purchased electric power, which is generated by a condensing turbine. The energy consumption in a salt manufacturing process was obtained by adding the generation energy in the back-pressure turbine, the evaporation energy in the No. 1 evaporator in multiple-effect evaporators, the condensing energy in the heater in the No. 1 evaporator and purchased energy. The energy consumption in a salt manufacturing process using the brine discharged from a reverse osmosis seawater desalinating plant was 80% of the energy consumption in the process using seawater. The optimum current density at which the energy consumption is minimized was 3 A/dm 2 for both electrodialyses of brine discharged from the reverse osmosis desalination plant and of seawater.